When an infusion is performed on a patient, or in other similar situations, an indwelling needle connected to an infusion line is made to puncture a blood vessel of the patient and the needle is left indwelling in the patient's blood vessel during the operation.
Such an indwelling needle (indwelling needle assembly) is composed of a hollow outer needle, an outer needle hub secured to a proximal end (base end) of the outer needle, an inner needle that is inserted in the outer needle and which has a sharp needle tip at a distal end (tip) thereof, and an inner needle hub secured to a proximal end of the inner needle. The outer needle hub is provided with a main pipe having a flow path that communicates with a lumen of the outer needle, and a side pipe having a branch flow path branching from the flow path of the main pipe. The infusion line is connected to the side pipe. (See, for example, Patent Document 1.)
When the indwelling needle punctures the patient's blood vessel, the puncturing operation is performed in a condition where the inner needle is inserted into the outer needle, and the needle tip of the inner needle protrudes from the distal end of the outer needle.
Once the needle tip of the inner needle has reached the inside of the blood vessel, blood flows into the inner needle through a distal portion thereof. The blood, in its course, flows through a hole formed in a side portion of the inner needle and into a flow path between the outer needle and the inner needle. The blood flows through the flow path, and then flows into the inside of the transparent outer needle hub. More specifically, the blood flows into the flow path of the main pipe, and further flows through the main pipe into the branch flow path of the side pipe (flashback). Consequently, it can be confirmed (visually checked) that the inner needle has captured (securely reached the inside of) the blood vessel.
After flashback is confirmed, the outer needle is advanced using the inner needle as a guide, and the outer needle is inserted into (allowed to puncture) the blood vessel.
Next, while grasping the outer needle hub, the inner needle is pulled out of the outer needle. Then, an infusion agent is administered through the infusion line, the side pipe and the main pipe of the outer needle hub, and the outer needle, which are in connection with one another.
However, the aforementioned conventional indwelling needle assembly has the following drawback. During a process in which, after the needle tip of the inner needle has reached the inside of the blood vessel, the blood passes through the flow path between the outer needle and the inner needle and flows into the flow path of the main pipe of the outer needle hub, and then through the main pipe into the branch flow path of the side pipe, bubbles (air) tend to remain in a part of the flow path of the main pipe that is located in the vicinity of the side pipe.    Patent Document 1: U.S. Pat. No. 6,749,588